Abstract Voluntary control of elbow extension significantly improves functional abilities for individuals with tetraplegia. As a result, surgical reconstruction of elbow extension via tendon transfer is considered a fundamental intervention that benefits the patient, even if other tendon transfers aimed at improving hand function are never performed. Presently, there are two common tendon transfer surgeries used to restore elbow extension following spinal cord injury. These are the posterior deltoid to triceps (PD-TRI) transfer and the biceps to triceps (BI-TRI) transfer. Both surgeries significantly improve voluntary elbow extension, although there is variability in the amount of voluntary control that is restored among patients. It remains unclear if this variability is related to the mechanical properties of the selected donor muscle, the procedures used by the surgeon, or difficulties associated with controlling the transferred muscle as an elbow extensor. This study will directly compare the performance of PD-TRI and BI-TRI tendon transfers with regard to: voluntary elbow extension strength, the ability to activate the transfer, and neural factors associated with voluntary and involuntary control of individual muscles. These comparisons will be made in functionally relevant postures and will provide fundamental information that will improve clinical understanding of the capacity of each of these two procedures to restore elbow extension. The fundamental hypothesis of this proposal is that an inability to maximally activate the transferred posterior deltoid and the transferred biceps significantly limits the elbow extension moment that can be produced. Our preliminary data suggests that approximately only 60% of the available transferred muscle strength can be recruited voluntarily. We further hypothesize that activation deficits change with posture and that the magnitude and nature of this change differs between the candidate transfers. Such changes could limit the workspace in which a transferred muscle could be used. Our first aim will quantify these activation deficits across three postures relevant to the use of these transfers in typical activities of daily living. Our second two aims will assess the cortical and spinal contributions to the ability to activate these muscles voluntarily. Together, the results from these aims will provide a greater understanding of the biomechanics associated with each potential donor muscle and the neural constraints on its use as an elbow extensor. These results will facilitate an understanding of the relative merits of the BI-TRI and PD-TRI surgeries and the neural constraints limiting their use. Importantly, the neurophysiological assessments also have important implications for developing appropriately targeted rehabilitation interventions aimed at improving the outcomes associated with these important surgical approached to the restoration of voluntary elbow extension.